In a flip-chip package, interconnect signals from an integrated circuit die connects to a package substrate through solder bumps (e.g., C4 bumps). As integrated circuits become more complex, more and more interconnect signals are required, and consequently, the number of solder bumps in a package also rises. With manufacturing processes moving to smaller geometries, the density of solders bumps increases dramatically, which severely reduces the amount of space left in the package to route the ever increasing number of interconnect signals. As a result, placement of the solder bumps is becoming exponentially more difficult.
An alternative packaging technique to a flip-chip package is a bumpless build-up layer (BBUL) package. Unlike a flip-chip package, the BBUL package does not use solder bumps to interconnect signals from the die to the package substrate. Instead, the BBUL package has build-up layers that are formed directly on the die. Interconnect layers are embedded in the build-up layers as part of the build-up process. By eliminating the need for solder bumps, a BBUL package allows the routing density to increase to accommodate the increasing number of interconnect signals.
Another benefit of the BBUL package is that package-on-package (PoP) pads can be formed on the top side of the package as part of the build-up process. The PoP pads allow for other integrated circuit devices such as memory devices and other electrical components such as decoupling capacitors to be attached to the top side of the BBUL package. This enables the placement of these other components to be much closer to the die to improve the performance of the integrated circuit. One type of PoP pad surface finish is a nickel-gold (Ni—Au) surface finish formed by an electrolytic plating process. The Ni—Au surface finish is formed by electrolytic plating a layer of Au onto an etch layer of copper (Cu), and then electrolytic plating a layer of Ni onto the layer of Au. Because this surface finish is formed by an electrolytic plating process, a manufacturer is required to have electrolytic surface finish equipment in order to manufacture the BBUL package.